Virtual memory protocol segmentation offloading

ABSTRACT

Methods and systems for a more efficient transmission of network traffic are provided. According to one embodiment, payload data originated by a user process running on a host processor of a network device is fetched by an interface of the network device by performing direct virtual memory addressing of a user memory space of a system memory of the network device on behalf of a network interface unit of the network device. The direct virtual memory addressing maps physical addresses of various portions of the payload data to corresponding virtual addresses. The payload data is segmented by the network interface unit across one or more packets.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/995,922, filed Jan. 14, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/568,499, filed Sep. 30, 2015, now U.S. Pat. No.9,319,490, which is a continuation of U.S. patent application Ser. No.13/853,326, filed Mar. 29, 2013, now U.S. Pat. No. 8,964,785, which is acontinuation of U.S. patent application Ser. No. 13/096,973, filed Apr.28, 2011, now U.S. Pat. No. 8,411,702, which is a continuation of U.S.patent application Ser. No. 12/254,931 filed Oct. 21, 2008, now U.S.Pat. No. 7,944,946, which claims the benefit of U.S. ProvisionalApplication No. 61/060,114 filed Jun. 9, 2008, all of which are herebyincorporated by reference in their entirety for all purposes.

COPYRIGHT NOTICE

Contained herein is material that is subject to copyright protection.The copyright owner has no objection to the facsimile reproduction ofthe patent disclosure by any person as it appears in the Patent andTrademark Office patent files or records, but otherwise reserves allrights to the copyright whatsoever. Copyright © 2008-2016, Fortinet,Inc.

BACKGROUND

1. Field

Embodiments of the present invention generally relate to network trafficacceleration. In particular, embodiments of the present invention relateto protocol segmentation offloading, such as Transmission ControlProtocol (TCP) segmentation offloading (TSO).

2. Description of the Related Art

FIG. 1 conceptually illustrates TSO processing as it is typicallyperformed. Large chunks of outbound network traffic, such as payloaddata 157, are typically broken down into smaller segments, such aspackets 170. This process is typically performed by the TCP protocol inthe host computer and is referred to as segmentation. When segmentationis performed on behalf of the host computer by separate hardware or aseparate processor, such as one associated with a network interfacecontroller (NIC), it is referred to as TCP packet SegmentationOffloading (TSO).

Conventional operating systems usually segregate virtual memory intokernel space 140 and user space 150. User mode applications, such asuser process 120, are forbidden from writing to or otherwise accessingkernel space 140. User space 150 is the memory area in which user modeapplications are permitted to operate.

Typically, TCP payload, such as payload data 157 is originated from auser process, such as user process 120. A kernel process 110 may createheader data 145 and store the header data 145 in kernel space 140 of asystem memory 130. TSO is used to increase system throughput anddecrease central processing unit (CPU) usage; however, in traditionalTSO implementations, in order to allow it to be physically addressed bya TCP segmentation unit 160, the payload data 157, which is stored inuser space 150, needs to be copied from user space 150 to contiguouskernel space 140 by the CPU (not shown) to create a payload data copy147. This movement of payload data from user space 150 to kernel space140 within system memory 130 is CPU intensive and reduces outbound TCPtraffic throughput.

Thus, there is a need in the art for improved outbound network trafficprocessing.

SUMMARY

Methods and systems are described for a more efficient transmission ofnetwork traffic. According to one embodiment, a method is provided forperforming segmentation offloading. Payload data originated by a userprocess running on a host processor of a network device is fetched by aninterface of the network device by performing direct virtual memoryaddressing of a user memory space of a system memory of the networkdevice on behalf of a network interface unit of the network device. Thedirect virtual memory addressing maps physical addresses of variousportions of the payload data to corresponding virtual addresses. Thepayload data is segmented by the network interface unit across one ormore packets.

Other features of embodiments of the present invention will be apparentfrom the accompanying drawings and from the detailed description thatfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example,and not by way of limitation, in the figures of the accompanyingdrawings and in which like reference numerals refer to similar elementsand in which:

FIG. 1 conceptually illustrates TSO processing as it is traditionallyperformed.

FIG. 2 conceptually illustrates TSO processing in accordance withvarious embodiments of the present invention.

FIG. 3 is an example of a system in which embodiments of the presentinvention may be utilized.

FIGS. 4A-4B depict exemplary virtual addressing mechanisms that may beused in relation to various embodiments of the present invention.

FIG. 5 is a flow diagram illustrating outbound TCP traffic processing inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Methods and systems are described for a more efficient transmission ofnetwork traffic. According to one embodiment, TSO processing does notrequire copying of payload data from user space to kernel space as aresult of allowing virtual memory to be directly addressed by or onbehalf of a network interface. Additionally, in one embodiment,scatter-gather functionality is provided, which allows data stored innon-continuous user space and/or kernel space and from which one or moreTCP packets are to be formed to be fetched and concatenated together. Invarious embodiments, buffer descriptors used by the scatter-gatherfunctionality can point to either physical memory or virtual memory.

For purposes of simplicity, various embodiments of the present inventionare described in the context of a TCP traffic segmentation. It is to benoted, however, that the TSO processing described herein, for example,may be implemented generically enough so as to be used for offloadingfragmentation of other transport layer protocols, or by doing InternetProtocol (IP) fragmentation for protocols that don't supportfragmentation themselves, such as Universal Datagram Protocol (UDP).Thus, embodiments of the present invention provide techniques that maygenerally increase outbound throughput of high-bandwidth networkconnections by reducing CPU overhead.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of embodiments of the presentinvention. It will be apparent, however, to one skilled in the art thatembodiments of the present invention may be practiced without some ofthese specific details. In other instances, well-known structures anddevices are shown in block diagram form.

Embodiments of the present invention include various steps, which willbe described below. The steps may be performed by hardware components ormay be embodied in machine-executable instructions, which may be used tocause a general-purpose or special-purpose processor programmed with theinstructions to perform the steps. Alternatively, the steps may beperformed by a combination of hardware, software, firmware and/or byhuman operators.

Embodiments of the present invention may be provided as a computerprogram product, which may include a machine-readable medium havingstored thereon instructions, which may be used to program a computer (orother electronic devices) to perform a process. The machine-readablemedium may include, but is not limited to, floppy diskettes, opticaldisks, compact disc read-only memories (CD-ROMs), and magneto-opticaldisks, ROMs, random access memories (RAMs), erasable programmableread-only memories (EPROMs), electrically erasable programmableread-only memories (EEPROMs), magnetic or optical cards, flash memory,or other type of media/machine-readable medium suitable for storingelectronic instructions. Moreover, embodiments of the present inventionmay also be downloaded as a computer program product, wherein theprogram may be transferred from a remote computer to a requestingcomputer by way of data signals embodied in a carrier wave or otherpropagation medium via a communication link (e.g., a modem or networkconnection).

TERMINOLOGY

Brief definitions of terms used throughout this application are givenbelow.

The terms “connected” or “coupled” and related terms are used in anoperational sense and are not necessarily limited to a direct connectionor coupling.

The phrases “in one embodiment,” “according to one embodiment,” and thelike generally mean the particular feature, structure, or characteristicfollowing the phrase is included in at least one embodiment of thepresent invention, and may be included in more than one embodiment ofthe present invention. Importantly, such phases do not necessarily referto the same embodiment.

If the specification states a component or feature “may”, “can”,“could”, or “might” be included or have a characteristic, thatparticular component or feature is not required to be included or havethe characteristic.

The term “responsive” includes completely or partially responsive.

FIG. 2 conceptually illustrates TSO processing in accordance withvarious embodiments of the present invention. It is to be understoodthat the present example is being described at an abstract level inconnection with a logical view of various functional units, including,hardware and software components. Thus, FIG. 2 does not necessarilyrepresent an actual physical view of the hardware and/or softwarecomponents depicted or their interconnections.

In contrast to the TSO processing illustrated by FIG. 1, in the presentexample, TCP payload data need not be copied from user memory space tokernel memory space and need not be stored in contiguous memory. Forpurposes of the present example, it is assumed (as is typically thecase) that the TCP payload is being originated by one or more userprocesses, such as user process 220. For simplicity, only one userprocess (user process 220), one network driver (network driver 215), onekernel process (kernel process 210) and one page table (page table 225)are shown and described in connection with the present example. It willbe appreciated by those skilled in the art that multiple user processes,multiple network drivers, multiple kernel process and multiple pagetables may be employed. For example, a page table may be used fortranslating virtual addresses to physical address for each user process.

According to the present example, a network driver 215, a kernel process210 and/or a user process 220 may affect the content of a system memory230 by storing data intended for Media Access Control (MAC) transmissionor information to facilitate such transmission. System memory 230 isshown containing multiple buffer descriptors 240, a header buffer 250, apage table 255 and multiple payload buffers (i.e., payload buffer₁ topayload buffer_(N)).

The page table 255 is a translation data structure, which will bedescribed further below, that allows interface hardware coupled to thesystem memory to directly address virtual memory to facilitate efficientTSO processing. According to one embodiment, the page table translationdata structure mimics the 32-bit Intel Architecture (IA32) page table.In such an embodiment and in the context of an IA32 system, the networkdriver 215 can re-use the system's native page table structure. In thecontext of other systems, the network driver 215 may construct thedesired page table structure from the native page table.

According to one embodiment, if payload buffers 260 are stored inphysically or virtually contiguous addresses, then the payload buffers260 can be addressed by a single buffer descriptor 240; otherwisemultiple buffer descriptors 240 are used to point to correspondingnoncontiguous payload buffers containing TCP payload data, which can bescatter-gathered. In the present example, a user process 220 mayoriginate TCP payload data to be transmitted across a network (notshown). The user process 220 will typically rely on the network driver215 to store the TCP payload data to system memory 230. In oneembodiment, network driver 215 can use multiple buffer descriptors 240(e.g., buffer descriptor₁ to buffer descriptor_(N)) to specify severalpayload buffers 260 (e.g., payload buffer1 to payload buffer_(N)), whichmay be stored in physically noncontiguous memory and across which theuser process originated TCP payload data may be distributed.Alternatively, a single buffer descriptor may be used to refer to asingle payload buffer storing the TCP payload data or the multiplebuffer descriptors may be limited to being associated with multiplepayload buffers stored in physically contiguous memory. Notably, inembodiments in which multiple payload buffers may be stored inphysically noncontiguous memory, a scatter-gathering capabilitydescribed further below may be provided to allow efficient retrieval ofthe noncontiguous payload buffers from user memory space.

A kernel process 210 may create an appropriate TCP template header (notshown) and store the TCP template header within a header buffer 250 inkernel memory space for later use in connection with forming packets290.

In one embodiment, each of the buffer descriptors 240 includes a PDBRfield 241, a buffer address field 242, an MSS field 243, a CMD field 244and a length field 245. Buffer descriptors 240 may be created by networkdriver 215 to point to a piece of memory (buffer) starting at an addressspecified by the buffer address 242 and having a number of bytesspecified by the length field 245. The MMS field 243 specifies a maximumsegment size (MSS) used by a TCP segmentation process 280 toappropriately segment the TCP payload data into an appropriate number ofpackets 290. The network driver 215 can use multiple buffer descriptorsto specify several buffers for one packet and scatter-gathering hardware270 can then fetch and concatenate them together to form a single packetfor MAC transmission.

Each of the buffer descriptors 240 can point to either physical memoryor virtual memory. In one embodiment, a virtual memory bit is providedwithin the CMD field 244 to indicate whether the address containedwithin the buffer address field 242 is a virtual or a physical address.As will be described further below, when the address contained withinthe buffer address field 242 is a virtual address, the PDBR field 241contains the address of the base of a page directory in the page table255 that will be used to translate the virtual address to a physicaladdress thereby allowing direct virtual memory addressing of user memoryspace to retrieve payload data from a corresponding payload buffer.

In one embodiment, the functionality of one or more of theabove-referenced functional units may be merged in various combinations.For example, the scatter-gathering module 270 and the TCP segmentationmodule 280 may be combined. Moreover, the various functional units canbe communicatively coupled using any suitable communication method(e.g., message passing, parameter passing, and/or signals through one ormore communication paths, etc.). Additionally, the functional units canbe physically connected according to any suitable interconnectionarchitecture (e.g., fully connected, hypercube, etc.).

According to embodiments of the invention, the functional units can beany suitable type of logic (e.g., digital logic, software code and thelike) for executing the operations described herein. Any of thefunctional units used in conjunction with embodiments of the inventioncan include machine-readable media including instructions for performingoperations described herein. Machine-readable media include anymechanism that provides (i.e., stores and/or transmits) information in aform readable by a machine (e.g., a computer). For example, amachine-readable medium includes, but is not limited to, read onlymemory (ROM), random access memory (RAM), magnetic disk storage media,optical storage media or flash memory devices.

According to one embodiment and as described further below, thescatter-gathering functionality is performed on behalf of a networkinterface of a network device by a bus/memory interface coupled to thesystem bus and the TCP segmentation processing is performed by thenetwork interface.

Returning to the role of the user process 220 briefly, it is worthnoting that in various embodiments locking and/or signaling techniquescan be used to communicate to user space processes when the payloadbuffers 260 are available to be reused and/or the associated memory canbe freed. In one embodiment, once a corresponding buffer descriptor iscreated by the network driver 215 for a particular payload buffer, thenetwork driver 215 can mark the pages as read-only to user spaceapplications. In this manner, if the application 220 writes to thebuffers again, an exception would be raised and the kernel would be ableto make a writable copy available to the user application 220. Inalternative embodiments, the user space process 220 can be made aware ofthe need to wait until the payload data from the payload buffer has sentbefore attempting to write to payload buffer again. For example, in thecontext of a Linux vmsplice( ) call, the user process 220 should notwrite to the page again until the page is no longer shared. In otherembodiments, synchronization may be performed at a higher level (e.g., ahigh-level protocol in which the other end acknowledges receipt of thepayload data).

FIG. 3 is an example of a network device 300 in which embodiments of thepresent invention may be utilized. In the present example, networkdevice 300 includes a network interface 350, a bus/memory interface 340,an interconnect bus, a general purpose processor 310 and a systemmemory. General purpose processor 320 may be any processor that istailored for executing software commands indicated by an operatingsystem. Thus, for example, general purpose processor may be, but is notlimited to the various processors currently found in personal computerssuch as those offered by Intel and AMD. Based on the disclosure providedherein, one of ordinary skill in the art will recognize a variety ofgeneral purpose processors that may be used in relation to differentembodiments of the present invention. In one embodiment, processor 320may be implemented as a semiconductor device such as, for example, aprogrammable gate array or an application specific integrated circuit.

Bus/memory interface 340 provides control for interconnect bus 330 andaccess to system memory 320. In particular embodiments of the presentinvention, interconnect bus 330 is a Peripheral Component Interconnect(PCI) bus, system memory 320 is a random access memory 330, andbus/memory interface 340 is a chipset currently available forcontrolling the PCI bus and providing access to system memory 330. Itshould be noted that interconnect bus 330 may be, but is not limited to,a PCI interface, a Peripheral Component Interconnect Extended (PCI-X)interface, a Peripheral Component Interconnect Express (PCIe) interface,or a HyperTransport (HT) interface.

As described with reference to FIG. 2, system memory 320 may have storedtherein, among other things, one or more buffer descriptors (e.g.,buffer descriptors 240), a TCP template header contained within a headerbuffer (e.g., header buffer 250), a page table, and one or more payloadbuffers (e.g., payload buffer₁ to payload buffer_(N)). The page table255 is a translation data structure, which will be described furtherbelow, that allows interface hardware coupled to the system memory todirectly address virtual memory to facilitate efficient TSO processing.

Depending upon the particular implementation, network interface 350 maybe a network interface unit (NIU), such as a network interface card(NIC), or other network interface device to allow network device 300 toconnect to an outside network. In one embodiment, network interface 350includes a network processor or other digital logic (not shown) to allowit to perform TSO processing to offload the general purpose processor310. In one embodiment, TSO processing may be performed by the networkprocessor. Alternatively, TSO may be performed in hardware, and thekernel just does a memory address conversion on each vector of ascatter-gather table.

In one embodiment, bus/memory interface 340 is configured to perform thescatter-gathering processing and capable of performing direct virtualmemory addressing of system memory 320 based on a page table translationdata structure stored therein or based on the most recently usedpage-directory and page-table entries in on-chip caches calledtranslation lookaside buffers or TLBs 345. In some embodiments of thepresent invention, the bus/memory interface 340 implements virtualaddressing only for accesses to TCP payload data contained withinpayload buffers stored in user memory space via a PCI bus. In suchcases, the bus/memory interface 340 only includes a TLB 345 for thesystem memory 320. As described below, such a TLB may include referencefor both 4-KByte pages and 4-MByte pages. Most paging may be performedusing the contents of the TLBs inside the same task. PCI bus cycles tothe page directory and page tables in system memory 320 need only beperformed when the TLBs 345 do not contain the translation informationfor a requested page. The TLBs 345 may be invalidated when apage-directory or page-table entry is changed between different tasks.

FIGS. 4A-4B depict exemplary virtual addressing mechanisms that may beused in relation to various embodiments of the present invention. Inparticular, FIG. 4A shows a hierarchy of a page directory 410 and a pagetable 430 utilized when mapping virtual addresses 400 to exemplary4-KByte pages 440. The entries in page directory 410, such as directoryentry 411, point to page table 430, and the entries in page table 430,such as page-table entry 431, point to pages 440 in physical memoryspecified by a particular physical address, such as physical address441.

Based on (i) a base address of the page directory 410, which may bespecified by a PDBR field 420 of a buffer descriptor as described aboveand (ii) a directory field 401, a table field 402 and an offset field403 of the virtual address 400, the bus/memory interface 340 may performdirect virtual memory addressing of user memory space to retrievepayload data from a corresponding payload buffer on behalf of a networkprocessor, for example, of the network interface 350.

A register (not shown) may be used to indicate when an associatedgeneral purpose processor has invalidated one or more entries of pagedirectory 410. Where such invalidation occurs, it is up to the networkinterface 350 and/or the bus/memory interface 340 to refresh the pagetable by accessing system memory 320.

FIG. 4B shows a process for using a page directory 460 to map a virtualaddress 450 to exemplary 4-MByte pages 480. The entries in pagedirectory 460, such as directory entry 461, point to 4-MByte pages 480in physical memory.

Based on (i) a base address of the page directory 460, which may bespecified by a PDBR field 470 of a buffer descriptor as described aboveand (ii) a directory field 451 and an offset field 452 of the virtualaddress 450, the bus/memory interface 340 may perform direct virtualmemory addressing of user memory space to retrieve payload data from acorresponding payload buffer on behalf of a network processor, forexample, of the network interface 350.

A register (not shown) may be used to indicate when an associatedgeneral purpose processor has invalidated one or more entries of pagedirectory 460. Where such invalidation occurs, it is up to the networkinterface 350 and/or bus/memory interface 340 to refresh the page tableby accessing system memory 320.

FIG. 5 is a flow diagram illustrating outbound TCP traffic processing inaccordance with an embodiment of the present invention. Depending uponthe particular implementation, the various process and decision blocksdescribed herein may be performed by hardware components, embodied inmachine-executable instructions, which may be used to cause ageneral-purpose or special-purpose processor programmed with theinstructions to perform the steps, or the steps may be performed by acombination of hardware, software, firmware and/or involvement of humanparticipation/interaction.

According to the present example, outbound TCP traffic processing beginsat decision block 510, in which it is determined if payload data ispresent. If so, processing continues with decision block 520; otherwiseprocessing branches back to decision block 510 until payload data ispresent. In one embodiment of the present invention, a network interface(e.g., network interface 350) or bus/memory interface (e.g., bus/memoryinterface 340) is interrupted on a periodic basis to trigger a check foran indication of available outbound payload data to be processed.Alternatively, the interrupts may be event driven and be receivedwhenever a network driver (e.g., network driver 215) stores a bufferdescriptor to system memory (e.g., system memory 230). Furthermore,interrupts may be received upon a payload buffer being stored to systemmemory. Such interrupts may be received using any interrupt scheme knownin the art including, but not limited to, using a polling scheme wherebus/memory interface or network interface periodically review aninterrupt register, or using an asynchronous interrupt port of a networkprocessor associated with the network interface. Alternatively oradditionally, network interface or bus/memory interface may proactivelyon an as needed basis when network interface has the processing andmemory resources to allow it to transmit additional data over a networkto which it is coupled. Based on the disclosure provided herein, one ofordinary skill in the art will recognize a variety of interrupt and/orpolling mechanisms that may be used in relation to different embodimentsof the present invention.

If payload data has been determined to be present, at decision block520, it is further determined whether the payload data is stored in avirtually addressed buffer (e.g., stored in user space) or a physicallyaddressed buffer (e.g., stored in kernel space). If the payload data isstored in a virtually addressed buffer, then processing continues withblock 540; otherwise processing branches to block 530. According to oneembodiment, the determination regarding whether the payload data isstored in a virtually addressed buffer is made with reference to abuffer descriptor associated with the payload data. For example, asdescribed above each payload buffer may have a corresponding bufferdescriptor containing information regarding one or more of an addressspecifying the starting address of the payload data in system memory, alength (e.g., in bytes) of the payload data, a maximum segment sizeapplicable to the payload data, a virtual memory indicator specifyingwhether the aforementioned address is a virtual or a physical addressand other information useful for locating, retrieving and otherwiseprocessing the payload data. In other embodiments, this determinationmay not be required as a convention could be established that allpayload data shall be stored in virtual memory.

At block 530, it has been determined that the payload data is stored ina physically addressed buffer. Therefore, the address of the payloaddata (specified within the corresponding buffer descriptor, for example)is recognized and used as a physical address.

At block 540, it has been determined that the payload data is stored ina virtually addressed buffer. Therefore, the address of the payload data(specified within the corresponding buffer descriptor, for example) istranslated to a physical address using a virtual addressing mechanism(e.g., a page table translation data structure, such as page table 255or one of those depicted in FIG. 4A or 4B). According to one embodiment,information is used from various portions of the address to perform pagetable walking to extract a physical address from a page tabletranslation data structure.

At block 550, the payload data is fetched from system memory using thephysical address determined in block 540 or the original buffer addressdetermined to be a physical address in decision block 520. According toone embodiment, a bus/memory interface performs both the direct virtualmemory addressing and the fetching from system memory on behalf of anetwork interface. In one embodiment, the fetching includes ascatter-gather process in which various portions of payload data havebeen stored in noncontiguous memory as part of multiple payload buffersand are pieced back together by retrieving the multiple payload buffers.In the most likely usage scenario, a packet consists of a physicallyaddressed buffer pointing to the (MAC+IP+TCP) header constructed by thekernel, followed by one or more virtually addressed buffers pointing tothe TCP payload originated by a user mode application. Advantageously,with the direct virtual memory addressing capability described herein,no user to kernel memory coping is needed thereby further increasingresource utilization of the general purpose processor of the networkdevice in the context of network traffic segmentation processing.

At block 560, TCP segmentation is performed. In one embodiment, afterfetching all the buffer data from both physically and virtuallyaddressed buffers (header and payload data, respectively), TCPsegmentation offloading, which segments the payload into multiplepackets to satisfy the maximum segment size, is performed. For example,one or more packets (e.g., packets 290) are formed by dividing up thepayload data gathered in block 550 among an appropriate number ofpackets based on the maximum segment size. According to one embodiment,the segmentation is performed by a network processor separate andindependent from the general purpose processor that originated thepayload data in an effort to increase system throughput and decreaseusage of the general purpose processor. Checksum offloading (CSO), whichcalculates the correct IP and TCP checksum for each segment can also beperformed. Advantageously, in accordance with embodiments of the presentinvention, both TSO and CSO can be performed without intervention fromthe general purpose processor of the network device.

While embodiments of the invention have been illustrated and described,it will be clear that the invention is not limited to these embodimentsonly. Numerous modifications, changes, variations, substitutions, andequivalents will be apparent to those skilled in the art, withoutdeparting from the spirit and scope of the invention, as described inthe claims.

What is claimed is:
 1. A method comprising: responsive to storage ofoutbound payload data within a user memory space of a system memory of anetwork device by a network driver of the network device on behalf of auser process running on a host processor of the network device,determining, by a bus/memory interface of the network device or anetwork interface unit of the network device, presence of the outboundpayload data, wherein the outbound payload data is to be delivered viaan Internet Protocol (IP) network to a destination via a transport layerprotocol and is distributed across a plurality of payload buffers withinthe user memory space; fetching, by the bus/memory interface on behalfof the network interface unit, the outbound payload data from theplurality of payload buffers by performing direct virtual memoryaddressing of the user memory space including mapping physical addressesof various portions of the outbound payload data to correspondingvirtual addresses and accessing a plurality of buffer descriptorscreated within the kernel memory space by the network driver, whereinthe plurality of buffer descriptors include: a first buffer descriptorincluding information indicative of a first starting address within thesystem memory of a first payload buffer of the plurality of payloadbuffers containing therein a first portion of the outbound payload data;and a second buffer descriptor including information indicative of asecond starting address within the system memory of a second payloadbuffer of the plurality of payload buffers containing therein a secondportion of the outbound payload data; and segmenting, by a networkprocessor of the network interface unit, the outbound payload dataacross one or more transport layer protocol packets.
 2. The method ofclaim 1, wherein: the first buffer descriptor further includesinformation indicative of whether the first starting address is virtualor physical; and the second buffer descriptor further includesinformation indicative of whether the second starting address is virtualor physical.
 3. The method of claim 1, wherein said segmenting comprisessegmenting the outbound payload data across one or more TransmissionControl Protocol (TCP) packets.
 4. The method of claim 1, wherein theplurality of buffer descriptors further contain information indicativeof a virtual memory base address of a page directory and wherein saidperforming direct virtual memory addressing of the user memory spacefurther comprises using directory entries of the page directoryidentified by the first starting address and the second startingaddress, respectively, to translate them to a first physical addresswithin the user memory space associated with the first payload bufferand a second physical address within the user memory space associatedwith the second payload buffer.
 5. The method of claim 1, wherein thefirst payload buffer and the second payload buffer are stored innoncontiguous locations of the user memory space.
 6. The method of claim5, wherein said performing direct virtual memory addressing of the usermemory space further includes a scatter-gather process in which theoutbound payload data is retrieved from the plurality of payloadbuffers.
 7. The method of claim 1, wherein said determining, by abus/memory interface of the network device or a network interface unitof the network device, presence of the outbound payload data comprisesthe bus/memory interface or the network interface unit checking for anindication of available outbound payload data to be processed responsiveto an interrupt.
 8. A non-transitory program storage device readable byone or more processors of a network device embodying a program ofinstructions executable by the one or more processors to perform amethod comprising: responsive to storage of outbound payload data withina user memory space of a system memory of a network device by a networkdriver of the network device on behalf of a user process running on ahost processor of the network device, determining, by a bus/memoryinterface of the network device or a network interface unit of thenetwork device, presence of the outbound payload data, wherein theoutbound payload data is to be delivered via an Internet Protocol (IP)network to a destination via a transport layer protocol and isdistributed across a plurality of payload buffers within the user memoryspace; fetching, by the bus/memory interface on behalf of the networkinterface unit, the outbound payload data from the plurality of payloadbuffers by performing direct virtual memory addressing of the usermemory space including mapping physical addresses of various portions ofthe outbound payload data to corresponding virtual addresses andaccessing a plurality of buffer descriptors created within the systemmemory by a network driver running on the host processor, wherein theplurality of buffer descriptors include: a first buffer descriptorincluding information indicative of a first starting address within thesystem memory of a first payload buffer of the plurality of payloadbuffers containing therein a first portion of the outbound payload data;and a second buffer descriptor including information indicative of asecond starting address within the system memory of a second payloadbuffer of the plurality of payload buffers containing therein a secondportion of the outbound payload data; and segmenting, by a networkprocessor of the network interface unit, the outbound payload dataacross one or more transport layer protocol packets.
 9. Thenon-transitory program storage device of claim 8, wherein: the firstbuffer descriptor further includes information indicative of whether thefirst starting address is virtual or physical; and the second bufferdescriptor further includes information indicative of whether the secondstarting address is virtual or physical.
 10. The non-transitory programstorage device of claim 8, wherein said segmenting comprises segmentingthe outbound payload data across one or more Transmission ControlProtocol (TCP) packets.
 11. The me non-transitory program storage deviceof claim 8, wherein the plurality of buffer descriptors further containinformation indicative of a virtual memory base address of a pagedirectory and wherein said performing direct virtual memory addressingof the user memory space further comprises using directory entries ofthe page directory identified by the first starting address and thesecond starting address, respectively, to translate them to a firstphysical address within the user memory space associated with the firstpayload buffer and a second physical address within the user memoryspace associated with the second payload buffer.
 12. The non-transitoryprogram storage device of claim 8, wherein the first payload buffer andthe second payload buffer are stored in noncontiguous locations of theuser memory space.
 13. The non-transitory program storage device ofclaim 12, wherein said performing direct virtual memory addressing ofthe user memory space further includes a scatter-gather process in whichthe outbound payload data is retrieved from the plurality of payloadbuffers.
 14. The non-transitory program storage device of claim 8,wherein said determining, by a bus/memory interface of the networkdevice or a network interface unit of the network device, presence ofthe outbound payload data comprises the bus/memory interface or thenetwork interface unit checking for an indication of available outboundpayload data to be processed responsive to an interrupt.